If you’ve spent at least one summer here in NYC as an endurance athlete or somewhere else similarly subtropical (yes! subtropical!) then you already know how miserable those first few weeks of consistent hot weather can feel. But have you ever wondered what exactly is going on, and why eventually it gets a little less miserable as the summer progresses? Let’s take a deeper dive into heat acclimation and find out!
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What ultimately happens as you acclimate to the heat
Three efficiencies occur within your body.
Your resting heart rate and heart rate during exercise decreases, improved cardiac output. This means more blood is being pumped through your body with each heartbeat. This increases your capacity for longer bouts of exercise in the heat!
Your body gets better at retaining fluids. You will start sweating sooner but you don’t lose as many electrolytes in your sweat. This means less excessive fluid loss AND since the sweat is less “salty” it evaporates faster. You body utilizes this evaporative heat loss to cool your body down. (providing it’s not super humid)
Overall you experience a decrease in core and skin temperatures during rest and exercise. Also! a lower rate of glycogen depletion.
How long does it take for your body to adjust?
For most people, 10-14 days (a minimum of 5-10 runs of an hour or more) but trained athletes adapt well to heat within 5–7 days while continuing to improve aerobic performance.
Sweating changes can take up to 10 days
How hot are we talking?
Ambient temperatures ≥77°F (25°C) impair performance at running distances ≥800 m – and that’s not even taking humidity into account!
You’re only going to acclimate so far
On an especially hot day, and as you acclimate, you will still experience:
Increases in skin blood flow as your body shunts blood from your muscles to the body surface (and into the environment) which in turn reduces skeletal muscle blood flow, oxygen and nutrient delivery to tissues and organs, and metabolic waste removal from working muscle. In other words, this will make your run feel more challenging and uncomfortable.
Decreased blood volume due to blood plasma fluid donation to help cool the body via sweating
Increase in sweat rate, which contributes to dehydration
Summary
Repeated exposure to exercise in the heat induces heat acclimation (HA), which improves thermoregulatory mechanisms, skeletal muscle metabolism, cardiovascular stability, and whole body thermo-tolerance
There is no single HA protocol that would elicit “optimal” adaptations in all athletes. Generally, athletes can fully heat acclimate with 10 consecutive days of exercise for 90 minutes in 86°F/30°C wet-bulb globe temperature
Key changes that your body makes with chronic, daily, heat exposure are: an increase in sweat rate and decrease in sweat electrolyte concentration, increased blood plasma volume, lower skin and core temperatures, lower heart rate during exercise, increased fluid and cardiovascular stability, and a decrease in the metabolic cost of work (exercise)
While you’re acclimating, decrease intensity of your run for the first few days and make sure to take in extra fluids and electrolytes! Hydrate early, hydrate often.
